Elevated requirements on the internal quality of strand-cast starting products such as, for example, slab ingots, cogged ingots or billets require that continuous-casting systems be equipped with elements that reduce the strand thickness (for example with appropriate rolls) and that preferably act in the area of the end solidification (so-called soft reduction). Here, for example, the downstream elements of the slab-ingot continuous-casting systems are equipped with position- and/or force-regulated adjustable hydraulic cylinders. On the other hand, in the case of billet systems or bloom systems the drivers downstream of the roll assemblies are used to reduce the strand thickness.
Strict requirements must be placed on the precision of the individual reduction steps in order to be able to achieve good reproducible quality results. This is especially true when the particular shaping elements are adjusted in a position-regulated manner. In this instance a tolerance for the position of the individual rolls of approximately 1/10 mm must be maintained. While it is technically readily possible to position the hydraulic cylinders for actuating the rolls with an appropriate precision, there are different possibilities of error in the case of other structural components required for transmitting the shaping forces.
In this regard the diameter of the strand-guide rolls and/or driver rolls used to shape the strand is especially problematic. The rolls are continuously in contact with the hot strand faces so they are subject to relatively high wear that can vary from roll to roll according to the environmental conditions (temperature, adjusting force, drive torque). In the extreme instance the roll wear can amount to a few millimeters of diameter. Furthermore, it varies over the length of roll. Thus, even the shape actually transmitted onto the metal strand varies, considered over a rather long time period and under conditions that are otherwise the same, so that the quality results obtained cannot be reproduced.
In order to avoid these problems, it is known to recalibrate the strand-guide rolls and/or drive rolls at predetermined intervals. To this end a test piece (test ingot) with known dimensions is run through the roll assemblies and precisely positioned customarily either in the system or on an external test stand. The roll pair or the driver is subsequently closed, so that the rolls rest on the test piece and a predetermined force is applied by the hydraulic cylinders. The measured cylinder position (and therewith also the roll position) can calculated from the known thickness of the test piece for a difference. The test pieces used for this can be designed according to the geometric relationships either as a separate part or can be mounted on the cold strand for the calibration.
EP 1 543 900 teaches a method of basic adjustment and monitoring of the roll nip of support roll assemblies or driver roll pairs in a continuous-casting machine for the casting of liquid metals in which the roll nip between opposite fixed and movable support rolls is measured and regulated via paired hydraulic piston-cylinder units with integrated electronic path- or position transmitters. The actual values are evaluated in a control circuit of the control for the forces to be periodically transmitted to the casting strand. For precise calibration of the roll nip a calibration piece comprising is moved through at least one roll pair of the open roll nip and clamped for a short time during the transport movement between two opposing support rolls, the positions of the piston-cylinder units associated with the clamping positions are stored with measuring technology, and the positions of the piston-cylinder units are corrected after a set-point/actual-value comparison before the start of casting or during a pause in casting.
A similar solution is described by EP 1 486 275, where a fixed roll assembly lower frame and a movable roll assembly upper frame that carry rolls are pressed against one another with a predetermined force of the hydraulic piston-cylinder units with paired distance pieces with a pre-calculated thickness outside of the roll nip between roll assembly upper frame and roll assembly lower frame, and that the associated actual values are determined by the path- or position detectors.
Similar solutions in which a test slab ingot used as a calibration piece is used are known, for example, from EP 0 047 919, DE 699 06 118, JP 0926 7159, JP 2003 112 240, KR 10200 1004 8624, JP 5700 1554, JP 0630 7937, and from JP 0308 6360.
All the above-described solutions have the disadvantage that the method of calibrating and measuring the nip between the rolls is relatively time-consuming, so that it is carried out only relatively infrequently for economic reasons. There is the problem here that in the interval between two calibrations or measurements the rolls of a roll assembly wear down further, so that the actual reduction of the thickness of the strand deviates from the adjusted values.